How to initialize two-dimensional arrays in Fortran

Question

In C you can easily initialize an array using the curly braces syntax, if I remember correctly:

int* a = new int[] { 1, 2, 3, 4 }; 

How can you do the same in Fortran for two-dimensional arrays when you wish to initialize a matrix with specific test values for mathematical purposes? (Without having to doubly index every element on separate statements)

The array is either defined by

real, dimension(3, 3) :: a 

or

real, dimension(:), allocatable :: a 

Answer 1

You can do that using reshape and shape intrinsics. Something like:

INTEGER, DIMENSION(3, 3) :: array array = reshape((/ 1, 2, 3, 4, 5, 6, 7, 8, 9 /), shape(array)) 

But remember the column-major order. The array will be

1   4   7 2   5   8 3   6   9 

after reshaping.

So to get:

1   2   3 4   5   6 7   8   9 

you also need transpose intrinsic:

array = transpose(reshape((/ 1, 2, 3, 4, 5, 6, 7, 8, 9 /), shape(array))) 

For more general example (allocatable 2D array with different dimensions), one needs size intrinsic:

PROGRAM main    IMPLICIT NONE    INTEGER, DIMENSION(:, :), ALLOCATABLE :: array    ALLOCATE (array(2, 3))    array = transpose(reshape((/ 1, 2, 3, 4, 5, 6 /),                            &     (/ size(array, 2), size(array, 1) /)))    DEALLOCATE (array)  END PROGRAM main